This invention relates to inspection and/or repair apparatus, and more particularly, a flexible shaft and probe used to inspect and/or repair the interior o longitudinally curved tubular members and passageways.
Hazardous environments often limit human access to particular locations. The inspection and repair of nuclear steam generators often requires remote accessing of critical components. Nuclear steam generators, for example, include a vertically oriented shell, a heat exchanger in the form of a bundle of u-shaped tubes disposed in the shell, a tube sheet that supports the tubes at the ends opposite the u-shaped curvature, and a dividing plate that cooperates with the tube sheet and forms a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle.
Radioactive primary fluid, once heated by circulation through the core of the nuclear reactor, enters the steam generator through the primary fluid inlet plenum. From there the primary fluid flows upwardly through first openings located in the u-shaped tubes near the tube sheet, then through the u-shaped tube curvatures, downwardly through second openings in the u-shaped tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, secondary fluid, known as feed water, is circulated around the exterior of the u-shaped tubes to cause heat transfer from the primary fluid to the feed water. A portion of the secondary fluid (feed water) is converted into steam. The walls of the u-shaped tubes and tube sheet isolate the secondary fluid from the radioactive primary fluid. It is thus important that the u-shaped tubes and tube sheet be maintained defect free so that no breaks occur in the u-shaped tubes or in the welds between the u-shaped tubes and the tube sheet that could lead to leakage and hence cause contamination of the secondary fluid by the radioactive primary fluid.
In order to either inspect or repair the u-shaped tubes or the tube sheet welds by way of access through the primary fluid inlet and outlet plena, manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena. However, the radioactive primary fluid often contaminates the inlet and outlet plena, thereby limiting the time in which working personnel may occupy this area. It is thus advantageous to perform inspection or repair of the u-shaped tubes and tube sheet without requiring the presence of working personnel. The following are exemplary of mechanisms known in the art that attempt to provide a solution to this problem, but all have shortcomings.
U.S. Pat. No. 4,303,884, issued to Malick, discloses an inspection probe or sensor for inspecting tubular members comprised of a plurality of axially oriented plastic strips which form the circumference of the probe, with each plastic strip having mounted thereon an eddy current coil. A rubber tube is disposed on the interior of the plastic strip so that when the rubber tube is inflated, the plastic strips and coils are forced outward into close contact with the tubular member so that the eddy current coils can detect flaws in the tubular member.
U.S. Pat. No. 4,668,912, issued to Junker, teaches an inflatable eddy current inspection probe having a molded central body with a plurality of eddy current coils and leads imbedded therein. The central body is expanded a will by introduction of water or air through a pressure hose connected to the central body to produce effective contact between the coils and the inside wall of the tube.
The above two types of inspection probes or sensors are part of a probe assembly that is pushed longitudinally from one end of the u-shaped tubes to the other by an element of the probe assembly known as the positioning shaft. However, positioning shafts known in the art are inadequate for pushing a probe assembly through u-shaped tubes having curves of relatively small radii. When the probe assembly encounters such sharp curves, frictional resistance to the probe assembly develops large lengthwise compressive forces in the positioning shaft causing it to buckle. This in turn generates side loads against the interior of the tube risking damage to the tube structure and increasing the frictional resistance to lengthwise movement of the assembly. Also, buckling of the positioning shaft makes the inspection and/or repair more difficult, consumes critical time, and often damages the probe assembly itself.
One type of probe assembly positioning shaft known in the art is a flexible polymer tube having a round cross-section and a hollow core. Carried within this core are a plurality of electrical instrumentation conductors and a flexible stainless steel emergency retrieval cable used to retrieve sensors that have become disconnected from the positioning shaft. However, known polymer positioning shafts suffer from an inability to resist buckling when subjected to typical compressive forces. Also, the polymer positioning shaft suffers from undesirable weakening when high temperatures are encountered in the heat exchanger tubes.
A second type of positioning shaft for probe assemblies is a helically wound metal casing having a continuous polymer coating. Some of these types of positioning shafts have a polymer coating that is not thick enough to resist buckling by compressive forces. Other positioning shafts of this type have a polymer coating that is so thick the positioning shaft cannot be bent sufficiently to negotiate bent tubes having curves of low radii.
Another known positioning shaft uses a helically wound metal casing, without a polymer coating. This type scores or scratches the interior of the heat exchanger tubes, and cannot be rolled for storage without risking the abrasion of the electrical instrumentation conductors of the probe assembly sensor.
A need thus exists for a positioning shaft of a probe assembly capable of traversing ben tubes having curvatures with small radii and similar small passageways having either a single sharp bend or a series of lesser bends or curves. A need also exists for a positioning shaft of the above type which will not buckle when subjected to lengthwise compressive forces, such as due to pushing on the shaft during positioning, and which will not scratch the interior of the passageway, and which, after removal, can be rolled for storage without abrading the electrical instrumentation conductors and elements of the sensor.